1. Field of the Invention
The present invention relates to an optical demultiplexer and an optical multi-demultiplexer, which are used in wavelength division multiplex communication, and also relates to an optical device using the optical demultiplexer or the optical multi-demultiplexer. More specifically, the present invention relates to an optical demultiplexer and an optical multi-demultiplexer, which are based on multi-mode interference (MMI), and also relates to an optical device using such an optical demultiplexer or optical multi-demultiplexer.
2. Description of the Background Art
In the field of optical communication, in order to increase communication capacity, there is currently proposed a system called a wavelength division multiplexing (WDM) scheme for multiplexing a plurality of signals into an optical signal, such that the plurality of signals are carried on different wavelengths of the optical signal, and for transmitting the optical signal via a single optical fiber. In the WDM scheme, important roles are played by an optical demultiplexer, which separates light into components of light of different wavelengths, and an optical multiplexer, which combines light of different wavelengths.
There is a conventional waveguide WDM optical demultiplexer or multi-demultiplexer which includes: an optical waveguide having a Y-shaped branching portion formed in a silicon substrate; a groove formed across the Y-shaped branching portion; and a dielectric multilayer film filter inserted into the groove (see, for example, pp. 1–4 and FIG. 3 of Japanese Patent Laid-Open Publication No. 63-33707).
Further, there is another conventional waveguide WDM optical demultiplexer or multi-demultiplexer using an arrayed waveguide grating (AWG) which includes: two input/output ends; two two-dimensional optical waveguides; and a plurality of elongated three-dimensional optical waveguides having different lengths (see, for example, pp. 1–6 and FIG. 1 of Japanese Patent Laid-Open Publication No. 2-244105).
Furthermore, there is still another conventional optical multi-demultiplexer for separating and combining light of a plurality of wavelengths using dual-wavelength optical multi-demultiplexers connected in series each including two MMI couplers and two parallel single-mode waveguides (see, for example, pp. 2–10 and FIG. 16 of Japanese Patent Laid-Open No. 2002-286952).
Further still, there is still another conventional optical multi-demultiplexer in which an input optical waveguide for allowing light of two different wavelengths to propagate therethrough is coupled to an input end of a multi-mode interference optical coupler, and the width and length of the multi-mode interference optical coupler are set such that components of the light of two different wavelengths are focused onto different locations on an output end of the multi-mode interference optical coupler (see, for example, pp. 2–5 and FIG. 11 of Japanese Patent Laid-Open Publication No. 8-201648; F. Rottmann, A. Neyer, W. Mevenkamp, and E. Voges, “Integrated-Optic Wavelength Multiplexers on Lithium Niobate Based on Two-Mode Interference”, Journal of Lightwave Technology, Vol. 6, No. 6, June 1988 (hereinafter, referred to as the “Document 1”); M. R. Paiam, C. F. Janz, R. I. MacDonald, J. N. Broughton, “Compact Planar 980/1550-nm Wavelength Multi/Demultiplexer Based on Multimode Interference”, IEEE Photonics Technology Letters, Vol. 7, No. 10, October 1995 (hereinafter, referred to as the “Document 2”); K. C. Lin and W. Y. Lee, “Guided-wave 1.3/1.55 μm wavelength division multiplexer based on multimode Interference”, IEEE Electronics Letters, Vol. 32, No. 14, 4 Jul. 1996 (hereinafter, referred to as the “Document 3”); and Baojun Li, Guozheng Li, Enke Liu, Zuimin Jiang, Jie Qin, and Xun Wang, “Low-Loss 1×2 Multimode Interference Wavelength Demultiplexer in Silicon-Germanium Alloy”, IEEE Photonics Technology Letters, Vol. 11, No. 5, May 1999 (hereinafter, referred to as the “Document 4”).
A conventional optical demultiplexer or multi-demultiplexer as disclosed in Japanese Patent Laid-Open Publication No. 63-33707 requires not only the optical waveguide but also an additional element, i.e., the dielectric multilayer film filter. Moreover, a process for producing such an optical demultiplexer or multi-demultiplexer requires subprocesses for forming the groove in the optical waveguide and inserting the dielectric multilayer film filter into the groove with high precision. The conventional optical demultiplexer or multi-demultiplexer separates or combines light by allowing light of different wavelengths to be reflected by or propagate through the dielectric multilayer film filter. Accordingly, it is necessary to provide an element for receiving separated light (e.g., a photodiode) and an element for emitting multiplexed light (e.g., a laser diode) at opposite ends of the conventional optical demultiplexer or multi-demultiplexer. Thus, in the case where the conventional optical demultiplexer or multi-demultiplexer is provided as a module including electric circuitry, it is necessary to form the electric circuitry in a position opposite to optical demultiplexer or multi-demultiplexer circuitry with respect to optical circuitry, i.e., optical waveguides, resulting in a module having a complicated structure.
Further, another conventional optical demultiplexer or multi-demultiplexer as disclosed in Japanese Patent Laid-Open Publication No. 2-244105 is developed for use in high density WDM of eight or more wavelengths, and therefore is expensive while being highly precise. Accordingly, when using such a conventional optical demultiplexer or multi-demultiplexer for low density WDM of about two to four wavelengths, the cost effectiveness is low as compared to the case of using the conventional optical demultiplexer or multi-demultiplexer as disclosed in Japanese Patent Laid-Open Publication No. 63-33707.
Furthermore, still another conventional optical multi-demultiplexer as disclosed in Japanese Patent Laid-Open Publication No. 2002-286952 is configured to separate and combine light of a plurality of wavelengths using the dual-wavelength optical multi-demultiplexer circuits connected in series each including two MMI couplers and two parallel single-mode waveguides. Accordingly, the structure of such a conventional optical multi-demultiplexer becomes complicated, and the optical path thereof is required to be lengthened, making it difficult to provide a compact optical multi-demultiplexer. Moreover, a significant loss of light is resulted from the lengthened optical path.
Further still, in still another conventional optical multi-demultiplexer of a first type, as disclosed in Document 1, a Y-like input waveguide and a Y-like output waveguide are respectively connected at the input and output sides of a multi-mode waveguide. In still another conventional optical multi-demultiplexer of a second type, as disclosed in Japanese Patent Laid-Open Publication No. 8-20164 and Documents 2 through 4, input and output waveguides are respectively connected at the input and output sides of the multi-mode waveguide so as to be parallel with each other. These two types of conventional optical multi-demultiplexers differ from each other in terms of shapes of the input and output waveguides. However, both types are alike in that the shape of the multi-mode waveguide is designed such that components of light of two different wavelengths are focused at (and outputted from) their respective output positions (i.e., output waveguide connecting positions as described later) determined in such a manner as to allow one wavelength to be in a bar-coupled state, while allowing the other wavelength to be in a cross-coupled state, thereby allowing the powers of the components to be maximized.
Accordingly, in both types of conventional optical multi-demultiplexers, although the power of light at each wavelength is maximized at the output position, an extinction ratio, which is a ratio between powers of light of a desired wavelength and another wavelength, is not always maximized at the output position. This is because distribution of the power of light at an output end is determined by the width of a fundamental mode at the wavelength of the light, and a position at which the power of light is maximized or minimized (hereinafter, referred to as the “maximum light power position” or “minimum light power position”) moves outwards within the multi-mode waveguide as the wavelength becomes longer. That is, at each output position, the power of light of one wavelength desired to be outputted is maximized, while the power of light of the other wavelength to be cut off is not minimized, and therefore the extinction ratio is not maximized at the output positions of a conventional optical multi-demultiplexer of any one of the types described above.
The term “extinction ratio” as used herein refers to a ratio which indicates the power of light of a desired wavelength to be outputted at one output position with respect to the power of light of a wavelength to be cut off at the same output position. Note that Documents 2 through 4 present the concept of the “ratio between powers of light” (i.e., the “contrast” or the “extinction ratio”) which indicates a ratio between powers of light at the same wavelength in different output positions. Thus, the concept as presented by Documents 2 through 4 is completely different from the concept of the extinction ratio as described herein which indicates the ratio between powers of light of different wavelengths in the same output position.
It depends on the use of the demultiplexer or multi-demultiplexer whether prime importance is placed on the ratio between powers of light of different wavelengths in the same output position or on the ratio between powers of light at the same wavelength in different output positions. If the demultiplexer or multi-demultiplexer is used for simply separating two wavelengths in one direction, prime importance may be placed on the ratio between powers of light at the same wavelength in different output positions as in the case of Documents 2 through 4. However, in the great majority of cases, e.g., as in the case of bidirectional communication, a transmitting device, as well as a light receiving device, is actually provided at the output side of the optical demultiplexer. Therefore, it is not practical to employ the conventional optical demultiplexer or multi-demultiplexer which is limited to the use for simply separating two wavelengths in one direction. In the case of using such a conventional optical demultiplexer or multi-demultiplexer, light of a wavelength other than a desired wavelength enters the transmitting device, leading to a malfunction or performance degradation of the conventional optical demultiplexer or multi-demultiplexer. Particularly, in the case of bidirectional WDM transmission, when light of a wavelength different from the transmitting wavelength enters a transmitting and light emitting section, such as a laser, a critical problem might be caused. Therefore, prime importance should be placed on the ratio between powers of light of different wavelengths in the same output position, rather than on the ratio between powers of light at the same wavelength in different output positions.
However, the wavelength dependence of the output position at the output end of the multi-mode waveguide is significantly influenced by the width of the multi-mode waveguide. As in the case of Japanese Patent Laid-Open Publication No. 8-201648 and Documents 2 through 4, when the width of the multi-mode waveguide is narrow, e.g., 5 μm to 12.6 μm, the width of the fundamental mode of a wavelength is also narrow. Accordingly, the distribution of the power of light at each output position on the output end is such that the maximum light power position of a desired wavelength to be outputted is close to the minimum light power position at a wavelength to be cut-off. Accordingly, a satisfactory extinction ratio can be obtained at the maximum light power position of the desired wavelength. As a result, there has been no necessity to discuss the issue concerning the position where the extinction ratio is maximized (hereinafter, referred to as the “maximum extinction ratio position”).
Further, in Japanese Patent Laid-Open Publication No. 8-201648 and Documents 1 through 4, it is not assumed that light of a wavelength different from the transmitting wavelength enters a transmitting and light emitting section, such as a laser, of a WDM transmitting/receiving module, and therefore the extinction ratio is not considered as an issue of further improvement. However, in the case where the width of the multi-mode waveguide is equal to or more than about 20 μm, there appears a remarkable difference between the maximum light power position of the desired wavelength and the maximum extinction ratio position, and therefore the issue concerning the maximum extinction ratio position can not be ignored.